Method for producing expanded or expandable polyolefin particles

ABSTRACT

The invention relates to a process for the production of expanded or expandable polyolefin beads by impregnating polyolefin granules with a volatile blowing agent in suspension under pressure. As suspension aid, use is made of a mixture of a water-insoluble inorganic compound and a surfactant, and the impregnation is carried out in the presence of a surface-active polymer carrying carboxyl or carboxylate groups.

The present invention relates to a process for the production ofexpanded or expandable polyolefin (PO) beads by impregnating PO granuleswith a volatile blowing agent in suspension under pressure at elevatedtemperature.

Said process is known per se and is operated on a large industrialscale. According to EP-A 53 333, EP-A 123 144 and DE-A 34 31 245 or DE-A199 50 420, the blowing agents employed are organic compounds, such ashalogenated hydrocarbons, for example dichlorodifluoromethane, orhydrocarbons, for example butane. The suspension medium is water, whichcan also be used as a mixture with alcohols. The suspension alwayscontains a suspension aid, which is intended to prevent the PO granulesfrom caking together or agglomerating in the suspension. Suitablesuspension stabilizers are water-insoluble inorganic compounds, such asalkaline earth metal carbonates, phosphates or oxides, which areemployed together with an ionic surfactant, for example a sulfonate.

However, this stabilizer system has a severe disadvantage: the finelydivided solid deposits on the polymer surface and thus prevents thebeads fusing together during subsequent conversion into moldings. Thesolid must therefore be removed from the surface before processing,which is effected in practice by acid washing. This results to aconsiderable pollution of wastewater by the acid and the stabilizersystem used.

It is an object of the present invention to overcome this disadvantage.

We have found that this object is achieved by addition of a polymercontaining carboxyl or carboxylate groups during the impregnation orafter the impregnation.

The invention therefore relates to a process for the production ofexpanded or expandable PO beads by impregnating PO granules with avolatile blowing agent in aqueous suspension in the presence of awater-insoluble inorganic compound in combination with a surfactant assuspension aid at elevated temperature under pressure, and theoptionally cooling the beads, followed by decompression, in which theimpregnation is carried out in the presence of a polymer containingcarboxyl or carboxylate groups.

The invention furthermore relates to a process for the production ofexpanded or expandable PO beads by impregnating PO granules with avolatile blowing agent in aqueous suspension in the presence of awater-insoluble inorganic compound in combination with a surfactant assuspension aid at elevated temperature under pressure, and thenoptionally cooling the beads, followed by decompression, in which thebeads are washed with an aqueous solution of a polymer containingcarboxyl or carboxylate groups.

For the purposes of the present invention, polyolefins are

a) homopolypropylene,

b) random copolymers of propylene with from 0.1 to 15% by weight,preferably from 0.5 to 12% by weight, of ethylene and/or aC₄-C₁₀-α-olefin, preferably a copolymer of propylene with from 0.5 to 6%by weight of ethylene or with from 0.5 to 15% by weight of 1-butene, ora terpolymer of propylene, from 0.5 to 6% by weight of ethylene and from0.5 to 6% by weight of 1-butene, or

c) a mixture of a) or b) with from 0.1 to 75% by weight, preferably from3 to 50% by weight, of a polyolefin elastomer, for example anethylene-propylene block copolymer containing from 30 to 70% by weightof propylene,

d) polyethylene (PE-LLD, PE-LD, PE-MD or PE-HD) and

e) a mixture of the polyolefins mentioned under a) to d) (if desiredafter addition of phase promoters).

Preference is given to homopolymers and copolymers of propylene with upto 15% by weight, in particular from 1 to 5% by weight, of ethyleneand/or 1-butene.

The crystalline melting point (DSC maximum) of the polyolefins listedunder a) to e) is generally from 90 to 170° C. Their enthalpy ofmelting, determined by the DSC method, is preferably from 20 to 300 J/g,and the melt flow index MFI (230° C., 2.16 kp for propylene polymers and190° C., 2.16 kp for ethylene polymers) in accordance with DIN 53 735 isfrom 0.1 to 100 g/10 min.

The production of the expanded or expandable PO beads starts from POgranules, which preferably have a mean diameter of from 0.2 to 10 mm, inparticular from 0.5 to 5 mm. 100 parts by weight of these granules aredispersed in from 100 to 500 parts by weight of the suspension medium ina stirred-tank reactor. The preferred suspension medium is water, whichcan also be used as a mixture with organic liquids, for example ethanoland methanol. The suspension medium contains suspension aids, preferablyin amounts of from 0.01to 10% by weight, in particular from 0.1 to 10%by weight, particularly preferably from 0.5 to 8% by weight, based onthe PO granules. The water-insoluble inorganic compound employed is afinely divided metal oxide, metal carbonate or metal phosphate,particularly preferably tricalcium phosphate, calcium carbonate,magnesium pyrophosphate, aluminum oxide, zinc carbonate, magnesiumcarbonate, titanium dioxide or siliceous earth. This water-insolublecompound is employed in combination with an ionic surfactant, forexample a sulfonate or sulfate, or with a nonionic surfactant which hasa hydrophilic moiety and a hydrophobic moiety, for example an ethoxylateof fatty alcohols, oxo alcohols, alkyl phenols, amines or amides, or acopolymer of ethylene oxide and propylene oxide. Examples of suitablepolymers containing carboxyl or carboxylate groups are homopolymers of(meth)acrylic acid, copolymers of (meth)acrylic acid with maleic acid orof maleic acid with C₂-C₂₂-olefins, or alkali metal salts, in particularsodium salts, of these polymers, preferably having molar masses of from10³ to 2·10⁵ g/mol.

The blowing agents employed can be either organic liquids or inorganicgases, or mixtures thereof. Suitable liquids are halogenatedhydrocarbons, but preference is given to saturated, aliphatichydrocarbons, in particular those having 3 to 8 carbon atoms. Particularpreference is given to n-butane. Suitable inorganic gases are nitrogen,air, ammonia and carbon dioxide. For expandable PO beads, blowing agentshaving a relatively high boiling point, for example pentane, should beemployed.

The blowing agent is employed in amounts of, preferably, from 1 to 50%by weight, in particular from 3 to 30% by weight, based on 100 parts byweight of polymer. The blowing agent can be added before or duringheating of the reactor contents to the decompression temperature. Thisshould be from 40° C. to 20° C. above the crystalline melting point ofthe polyolefin. In the case of the preferred propylene polymers,temperatures of from 110° C. to 180° C., in particular from 120° C. to160° C., are used.

Depending on the amount and type of the blowing agent and on the levelof the temperature, a pressure of from 2 to 100 bar generally arises inthe reactor.

For the production of expanded PO beads, the reactor is decompressedwhen the decompression temperature has been reached, the decompressionadvantageously taking place in an intermediate tank in which a pressureof from 0.5 to 5 bar preferably prevails. During decompression of thereactor, the blowing agent-containing PO granules expand, and PO foambeads having a mean diameter of from 1 to 20 mm are formed.

For the production of expandable PO beads in accordance with DE-A 199 50420, the reactor contents are preferably cooled to room temperaturebefore the decompression.

The beads are separated off from the aqueous solution containing themajority of the suspension aid, washed and dried, advantageously usingwashing water without added acid.

In an alternative embodiment of the invention, the polymer containingcarboxyl or carboxylate groups is not added during the impregnation, butinstead the beads separated off from the suspension medium are washedwith a preferably from 0.01 to 20% strength by weight aqueous solutionof the polymer.

Expandable PO beads can be stored for several hours at room temperature,even for several weeks in sealed containers, without significant loss ofblowing agent occurring. These beads can be expanded in a conventionalmanner in pressure prefoamers to give PO foam beads.

The bulk density of the PO foam beads is from 5 to 400 g/l, inparticular from 10 to 300 g/l. The foam beads are predominantlyclosed-cell and have from 2 to 3000 cells/mm², preferably from 5 to 2500cells/mm², in particular from 10 to 1500 cells/mm².

The PO foam beads can be fused by conventional methods using steam orhot air to give foam moldings which are used in the automobile,packaging and leisure industries.

EXAMPLE

6000 parts of a propylene-ethylene random copolymer having an ethylenecontent of about 2% by weight, 400 parts of calcium carbonate, 8 partsby a block copolymer of ethylene oxide and propylene oxide (molar mass8000 g/mol, ethylene content 80%) and 1 part of a copolymer of maleicacid and acrylic acid (Na salt, molar mass 50000 g/mol, SOKALAN CP 7from BASF) are dispersed in 14000 parts of water in a closedstirred-tank reactor. After addition of 1400 parts of butane, thereactor is heated to about 130° C. During this heating, the pressurerises to about 20 bar. The contents of the reactor are subsequentlydecompressed into an intermediate tank. After the liquid phase has beenlet out, the product is rinsed with water. The dried foam beads have abulk density of 17 g/l. They can be fused using steam to give stablemoldings.

I claim:
 1. A process for the production of expanded polyolefin beads byimpregnating polyolefin granules with a volatile blowing agent inaqueous suspension in the presence of at least 0.5% by weight, based onthe polyolefin granules, of a water-insoluble inorganic compound incombination with a surfactant as suspension aid at elevated temperatureunder pressure followed by decompression, which comprises carrying outthe impregnation in the presence of a polymer containing carboxyl orcarboxylate groups.
 2. A process for the production of expandablepolyolefin beads by impregnating polyolefin granules with a volatileblowing agent in aqueous suspension in the presence of at least 0.5% byweight, based on the polyolefin granules, of a water-insoluble inorganiccompound in combination with a surfactant as suspension aid at elevatedtemperature under pressure, and then cooling the beads, followed bydecompression, which comprises carrying out the impregnation in thepresence of a polymer containing carboxyl or carboxylate groups.
 3. Aprocess as claimed in claim 1 or 2, wherein the polymer is employed inamounts of from 10⁻³ to 10⁴% by weight, based on the water-insolubleinorganic compound.
 4. A process as claimed in claim 1 or 2, wherein thepolymer is a homopolymer of (meth)acrylic acid, a copolymer of(meth)acrylic acid with maleic acid or of maleic acid with aC₂-C₂₂-olefin, or an alkali metal salt of these polymers.
 5. A processas claimed in claim 1, wherein the water-insoluble inorganic compound isa finely divided metal oxide, metal carbonate or metal phosphate.
 6. Aprocess as claimed in claim 5, wherein the water-insoluble inorganiccompound is tricalcium phosphate, calcium carbonate, magnesiumpyrophosphate, aluminum oxide, zinc carbonate, magnesium carbonate,titanium dioxide or siliceous earth.
 7. A process according to thepreamble of claim 1, wherein, after separation from the suspensionmedium, the expanded or expandable polyolefin beads are washed with anaqueous solution of a polymer containing carboxyl or carboxylate groups.8. A process as claimed in claim 1, wherein the polymer is employed inamounts of from 10⁻² to 10²% by weight based on the water-insolubleinorganic compound.